Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.4.10

3.8.4.10

tadvā etadeko dvābhyāṃ vaṣaṭkaroti | adhvaryave ca yaścaiṣa upayajatyatha yadyajantamupayajati tasmādupayajo nāmātha yadupayajati praivaitajjanayati paścāddhyupayajati paścāddhi yoṣāyai prajāḥ prajāyante

Here now, one (Hotri) pronounces the Vashat for two,--for the Adhvaryu (who performs the after-offerings) and for him (the Pratiprasthâtri) who performs the by-offerings. And because he offers them by (in addition to) the offering (Adhvaryu), therefore they are called by-offerings. And in performing the by-offerings, he produces (offspring), since he performs the by-offerings behind (the altar), and from behind offspring is produced from woman.